1. Field of the Invention
The present invention relates to a radiation image detecting device comprising a dose measuring sensor for an exposure control of a radiation image, and a radiation imaging system using this device.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using radiation, e.g., X-ray is known. The X-ray imaging system comprises an X-ray generating device for generating X-ray and an X-ray imaging device for producing an X-ray image of an object (patient) with X-ray which penetrated the object. The X-ray generating device includes an X-ray source for irradiating X-ray toward the object, a source controller for controlling the X-ray source, and an emission switch for inputting a command to the source controller so as to activate the X-ray source. The X-ray imaging device includes an X-ray image detecting device for detecting the X-ray image based on X-ray which penetrated the object, and a console for controlling the X-ray image detecting device and for saving and displaying the X-ray image.
As the X-ray image detecting device, one with an image detection section (which is called a flat panel detector (FPD)) is in widespread use. The image detection section has a detection panel having imaging surface where pixels which accumulate electric charge according to an incident quantity of X-ray was arranged into a matrix, and a circuitry part for driving the detection panel. The image detection section detects an X-ray image as electric signals. The detection panel detects an X-ray image of an object by accumulating electric charge of each pixel and converting the accumulated electric charge into a voltage signal in a signal processing circuit. The detected X-ray image is output as digital image data. There also exists a portable type of X-ray image detecting device, which is called an electronic cassette.
In X-ray photography, to reduce an influence of scattered radiation which occurs when X-ray penetrates an object, an anti-scatter member called a grid may be located between the object and the X-ray image detecting device for the photography. The grid is formed of alternate arrangement of X-ray absorbing sections and X-ray transmitting sections each of which is formed into a slim strip shape. The X-ray absorbing section is made of a material such as lead, which absorbs and hardly transmits X-ray. The X-ray transmitting section is made of a material such as aluminum, which is capable of transmitting X-ray. Since the X-ray absorbing sections and the X-ray transmitting sections are arranged alternately in a single direction, a striped pattern is formed by these sections. By placing the grid between the imaging surface of the detection panel and an object, most of the scattered radiation is absorbed in the X-ray absorbing section in the grid before arriving at the imaging surface. Accordingly, an image having high contrast with a little influence of scattered radiation can be obtained. The grid is attached to an imaging stand, a housing of the X-ray image detecting device, or the like.
In addition, as one of items to classify a type of grid, there is a grid density representing the number of X-ray absorbing sections per unit width. For example, there are various types of grids having a grid density in a range of 26 lines/cm-100 lines/cm. In case that the grid density is 40 lines/cm (4 lines/mm), the grid pitch becomes 250 μm. The grid pitch is the width of one set of the X-ray absorbing section and the X-ray transmitting section.
Some of the X-ray image detecting device has an automatic exposure control (AEC) function. In the AEC, irradiation of X-ray from the X-ray source is stopped and exposure of an X-ray image is controlled when a dose of irradiated X-ray from the X-ray source reached the predetermined emission stop threshold value (e.g., U.S. Pat. Nos. 6,952,465 and 6,944,266). Such the X-ray image detecting device is provided with dose measuring sensors for detecting a dose of X-ray penetrated through an object and outputting a signal depending on the dose.
The U.S. Pat. No. 6,952,465 describes an X-ray image detecting device which provides dose measuring sensors of a stripe shape with the length for 500 pixels separately from pixels in an imaging surface of a detection panel. In the U.S. Pat. No. 6,952,465, the dose measuring sensors are arranged so that a longitudinal direction (direction of the stripe) of the dose measuring sensors of the stripe shape and a course of a stripe of a grid become non-parallel (for example, at right angles). Accordingly, even in case that a gap occurs in the geometric layout of the grid and the dose measuring sensors (relative positional relations of the grid and the dose measuring sensors), fluctuation of output value of the signal which the dose measuring sensor outputs can be suppressed to perform stable AEC.
The dose measuring sensors are provided in the imaging surface. Accordingly, production errors and installation backlash of the grid cause position gaps in the geometric layout with the grid and the dose measuring sensors. Each of the X-ray absorbing section and the X-ray transmitting section of the grid has a width of μm order. Accordingly, production errors and installation backlash of the grid easily make position gaps between the grid and the dose measuring sensors, in the degree of about the width of one of the X-ray absorbing sections and the X-ray transmitting sections. When position gaps in the geometric layout with the grid occur, an incident quantity of X-ray to the dose measuring sensors fluctuates even if an exposure dose of X-ray is the same. Accordingly, the output value of the dose measuring sensors fluctuates. A fluctuation range of the output value of the dose measuring sensors becomes greatest when the direction of the stripe of the grid is parallel to the direction of the stripe of the dose measuring sensors.
In case that the direction of the stripe of the grid is parallel to the direction of the stripe of the dose measuring sensors, the dose measuring sensor of the stripe shape may be covered by the X-ray absorbing section across the whole of the longitudinal direction, or may be covered by the X-ray transmitting section in the same way. When the whole area of the dose measuring sensor hides behind the X-ray absorbing section, an incident quantity of X-ray decreases in the whole area of the dose measuring sensor. Accordingly, output value of the dose measuring sensor is minimized. Conversely, even if an exposure dose of X-ray is the same, when the whole area of the dose measuring sensor hides behind the X-ray transmitting section, an incident quantity of X-ray increases in the whole area of the dose measuring sensor. Accordingly, output value of the dose measuring sensor is maximized. In this way, when the direction of the stripe of the dose measuring sensors and the direction of the stripe of the grid are collimated, a fluctuation range of the output value of the dose measuring sensors caused by the position gap of the geometric layout of the grid and the dose measuring sensors increases.
In consideration of this problem, in the U.S. Pat. No. 6,952,465, dose measuring sensors in a stripe shape are arranged in non-parallel with a direction of a stripe of a grid. Even when position gaps occur in the geometric layout of the grid and the dose measuring sensors, a part of the dose measuring sensor is always located behind the X-ray absorbing section, and the other part is located behind the X-ray transmitting section. In this configuration, an incident quantity of X-ray relatively lowers in apart of the dose measuring sensor. However, since an incident quantity of X-ray relatively increases in the other part of the dose measuring sensor, the output value of the dose measuring sensor is equalized. Therefore, in comparison with the case that the stripe direction of the dose measuring sensors are parallel to the stripe direction of the grid, a fluctuation range of the output value of the dose measuring sensors caused by the position gap of the geometric layout of the grid and the dose measuring sensors can be reduced. Accordingly, stable AEC can be realized.
In an embodiment of the U.S. Pat. No. 6,952,465, a size of pixels is 105 μm×105 μm, and dose measuring sensors have a length for 500 pixels. Accordingly, the length of the dose measuring sensors is about 105 μm×500=52,500 μm (approximately 50 mm). The dose measuring sensors replace some pixels or are placed between adjacent pixels. In case that the dose measuring sensors are placed between pixels, a size of pixels adjacent to the dose measuring sensors is reduced to make places for the dose measuring sensors. In this way, the plural dose measuring sensors are placed in predetermined areas.
In addition, in an X-ray image detecting device described in the U.S. Pat. No. 6,944,266, as a substitute for dose measuring sensors of a stripe shape, a part of pixels in an imaging surface is assigned as measuring pixels functioning as dose measuring sensors (referred to as AEC pixels in U.S. Pat. No. 6,944,266). The pixels of U.S. Pat. No. 6,944,266 can perform so-called non destructive read-out, in which output value is retrieved with holding the accumulated electric charge. The measuring pixels also can perform non destructive read-out.
In the X-ray image detecting device of the U.S. Pat. No. 6,944,266, the measuring pixels are placed in an imaging surface. Accordingly, like the U.S. Pat. No. 6,952,465, output values of the measuring pixels fluctuate by the position gap of the geometric layout of a grid and the measuring pixels. To deal with the problem that output value of the each measuring pixel fluctuates in the U.S. Pat. No. 6,944,266, a calibration of the output value of the each measuring pixel is carried out.
Specifically, in the U.S. Pat. No. 6,944,266, before object photography, X-ray is uniformly irradiated to the imaging surface to which the grid was attached, so as to obtain a gain image representing output value of the each measuring pixel in the imaging surface. The gain image reflects a fluctuation of the output value of the each measuring pixel to which the grid was attached. And in AEC for object photography, an output value of the each measuring pixel is corrected by applying calibration based on the gain image to the output value output from the each measuring pixel. The output value of each measuring pixel fluctuates by not only the type of the grid such as the grid density but also imaging conditions including a dose of X-ray, radiation quality depending on tube voltage, and so on. In addition, even in case that the type of the grid and imaging conditions are the same, since production errors and installation backlash of the grid cause position gaps in the geometric layout with the grid and the measuring pixels, the acquisition of gain image is carried out for every photography.
In the U.S. Pat. No. 6,952,465, spaces for the dose measuring sensors of stripe shape are secured by substitution of pixels and reduction of the pixel size. Accordingly, in photographed X-ray image, the density of area where dose measuring sensors are located decreases. Since density difference between the area where the dose measuring sensors are located and the adjacent area, a line-formed density step appears. Since the length of the dose measuring sensor of stripe shape is approximately 50 mm which is visible by human eye, the density step of the X-ray image becomes a degree highly visible by human eye. The U.S. Pat. No. 6,952,465 discloses that dose measuring sensors are considered to be defect pixels and a defect correction is carried out to cancel such a density step. However, to carry out the defect correction, an additional process to prepare correction data is required. Furthermore, it is difficult to cancel the defect to the degree that the defect is not visible even if the defect correction is carried out, because the dose measuring sensor has the large size of approximately 50 mm. Accordingly, there is a concern that the quality of the X-ray image will decrease.
The U.S. Pat. No. 6,944,266 uses a part of the pixels as the measuring pixels, and output value of the measuring pixel is obtained through non destructive read-out. Accordingly, the density step such as in the U.S. Pat. No. 6,952,465 does not occur in the X-ray image. Therefore there are not the problem that an additional process is required for the defect correction and the problem of quality deterioration of the X-ray image. However, the X-ray image detecting device of the U.S. Pat. No. 6,944,266 must acquire a gain image for each photography. This brings another problem that it should take time and require additional process for obtaining the gain image. Furthermore, appropriate correction is not possible even if the acquired gain image is used, in case that position gaps in the geometric layout with the grid and the measuring pixels are occurred by a shock or other causes before performing the object photography after acquiring the gain image.